An electric double layer capacitor is a charge accumulation device utilizing an electric double layer formed at the interface of polarizable electrodes and an electrolytic solution.
When the electrolytic solution used in an electric double layer capacitor possesses a high coefficient of viscosity or a low electric conductivity, the internal resistance of the electric double layer capacitor increases thereby decreasing the voltage of the capacitor during charging and discharging. Therefore, an electrolytic solution used in an electric double layer capacitor is required to possess a low viscosity and a high electric conductivity. In addition, the electrolytic solution is required to be durable over a long period of time.
Conventionally, in view of ensuring long term durability, electrolytic solution for use in electric double layer capacitors comprising a quaternary ammonium salt such as triethylmethylammonium tetrafluoroborate (hereinafter referred to as “TEMA-BF4”) dissolved in propylene carbonate (hereinafter referred to as “PC”) has been used (e.g. Japanese Patent Application Laid-open No. 2000-114105).
However, due to a high coefficient of viscosity of about 2.5 mPa·s at 30° C. possessed by the PC solvent, the electrolytic solution in which an electrolyte is dissolved in this solvent has a problem of a high coefficient of viscosity and a low conductivity, resulting in a large internal resistance of the electric double layer capacitor produced using this electrolytic solution.
For this reason, an electrolytic solution for an electric double layer capacitor having a low coefficient of viscosity, excellent low-temperature characteristics, and high long-term reliability has been desired.
An object of the present invention is to provide an electrolytic solution for use in an electric double layer capacitor possessing a low coefficient of viscosity and excellent low-temperature characteristics, i.e. exhibiting a high conductivity without coagulating over a wide temperature range including a low temperature range and having superior long-term reliability, and to provide an electric double layer capacitor using this electrolytic solution.